Method for joining rhenium to niobium

ABSTRACT

A method for joining rhenium to columbium and columbium based alloys that minimizes the formation of brittle intermetallics. First, a layer of ruthenium is applied to both the rhenium and columbium and then heated until the ruthenium diffuses with the rhenium and columbium respectively. A braze foil is place between the two outer ruthenium layers and the assembly is heated in a range of 50° C. to 100° C. above the liquidus of the braze, thereby joining the rhenium to the columbium. Alternatively, the ruthenium coated surfaces are placed in contact and diffused together under a compressive load of at least 1000 psi.

This application is a division of application Ser. No. 08/655,036, filed May 29, 1996 now U.S. Pat. No. 5,704,538.

TECHNICAL FIELD

This invention relates generally to bonding processes and in particular to a process for bonding rhenium to niobium and columbium based alloys.

BACKGROUND OF THE INVENTION

Currently, mono-propellant and bi-propellant radiation cooled rockets use nozzles and thrust chambers made of a niobium alloy, (Nb₁₀₃), covered with a fused silica coating for oxidation protection. These nozzles and thrust chambers generally can withstand temperatures as high as 2400° F. To increase the operating temperatures of these rockets, it has been proposed to combine the niobium nozzles with rhenium thrust chambers. When conventional joining methods, such as welding and brazing, are used to join rhenium to niobium, brittle intermetallics form. An intermetallic is a compound formed when two metals ionically bond. Because of the brittleness of the intermetallic, cracks can form and lead to the failure of the joint between the rhenium and niobium.

Accordingly there is a need for a method of joining rhenium to niobium that minimizes the formation of intermetallics.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of joining rhenium to niobium that minimizes the formation of intermetallics.

The present invention achieves this object by providing a method for joining rhenium to niobium that uses an intermediate layer of ruthenium which has a limited intermetallic formation characteristic. The ruthenium is coated onto both the rhenium and niobium and then exposed to high temperature to diffuse the ruthenium with the rhenium and niobium respectively. A braze foil is then placed between the two ruthenium outer layers and the assembly is heated in a range of 50° C. to 100° C. above the liquidus of the braze, thereby joining the rhenium to the niobium. Alternatively, the two ruthenium layers can be diffused together during the same thermal cycle in which the ruthenium is diffused with the rhenium and niobium.

BRIEF DESCRIPTION OF THE DRAWING

The sole figure is a cross-sectional view of an article formed by a method contemplated by the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A method for bonding rhenium to niobium includes the following steps. An article either made of, or having a portion of, or a coating of pure rhenium is to be bonded to an article either made of, or having a portion of, or a coating of pure columbium or alloys thereof. The alloy is preferably at least 95% niobium composition. An example of such an alloy is Nb₁₀₃.

The rhenium is cleaned in a nonpolar solvent and then immersed into a solution of water and nitric acid so as to slightly etch the surface of the rhenium. This step also removes oxides from the surface. After cleaning, a layer of pure ruthenium is electroplated to the rhenium. The thickness of the ruthenium is preferably between 0.001 inches to 0.003 inches. The ruthenium is then diffused into the rhenium by heating in a vacuum (pressure less than or equal to 5×10⁻⁵ torr), to a temperature between 1200° C. and 1900° C. for 1 to 4 hours (1800° C. for 1 hour is preferred) to obtain a mixed layer of ruthenium/rhenium which is typically 0.0007 inch thick. This layer provides a solid solution metallurgical interface which has a coefficient of thermal expansion between that of the rhenium and the ruthenium and is ductile in character allowing for deformation due to mismatch conditions which may exist during the brazing steps to follow.

The niobium is cleaned in a nonpolar solvent, and after cleaning, a layer of pure ruthenium is electroplated to the niobium. The thickness of the ruthenium is preferably between 0.001 inches to 0.003 inches. The ruthenium is then diffused into the niobium by heating in a vacuum, (pressure less than or equal to 5×10⁻⁵ torr), to a temperature between 1560° C. and 1850° C. for 1 to 4 hours (1800° C. for 1 hour is preferred) to obtain a layer of mixed niobium/ruthenium typically 0.0005 inch thick. This layer provides a solid solution mixture which is ductile in character and allows for deformation of mismatch conditions which may exist during the braze steps that follow. This layer has a coefficient of thermal expansion between that of niobium and ruthenium.

Alternatively, for both rhenium and niobium the ruthenium can be applied by wet application of an alcohol dissolved ruthenium salt.

The niobium and rhenium are assembled with a braze foil in contact with their respective ruthenium electroplated surfaces to form a braze interface. The braze foil should have a thickness between 0.002 and 0.005 inch, and can be any gold-nickel, gold-palladium-nickel, BAu-5 or any nickel-palladium combination. A compressive load in the range of 50 psi to 200 psi is then applied to the braze interface. In a manner familiar to those skilled in the art, the compressive load can be applied by various methods such as interference fit, external load, and/or thermal expansion difference fixture. The latter is a fixture that thermally expands less than the part to be brazed resulting in a net compressive force on the part when heated. Applying the compressive load is not essential to obtain the proper metallurgical bond but does minimize porosity and mechanical defects in the braze interface.

While maintaining the compressive load, the assembly is placed in a vacuum (pressure less than or equal to 1×10⁻⁵ torr) and heated to a temperature that is 50° C. to 100° C. above the liquidus of the braze alloy selected. This temperature should be held constant for between five to twenty minutes. The assembly is then cooled and the compressive load released.

In an alternative method, after the ruthenium has been electroplated onto the rhenium and niobium, the ruthenium coated surfaces are placed in contact and a compressive load of at least 1000 psi is applied and the assembly is heated in a vacuum (pressure less than or equal to 5×10⁻⁵ torr), to a temperature between 1750° C. and 1900° C. for at least one hour. The assembly is then cooled to ambient temperature and the compressive load removed. This process results in diffusion bonding of the ruthenium layers.

Referring to sole figure, with either process an article is formed comprising a layer of rhenium 10, a layer of niobium or a columbium based alloy 14, and a ruthenium bond layer 12 disposed between the layers 10 and 14. Importantly, the bond layer 12 is of a material that is different than the material of the layers 10 and 14. When the braze process described herein is used, the resulting bond layer 12 is comprised of a ductile layer of mixed rhenium-ruthenium, braze alloy-ruthenium and a ductile mixed layer of ruthenium-niobium. When the alternate diffusion process is used the resulting bond layer 12 is comprised of a ductile layer of mixed rhenium-ruthenium, a pure layer of ruthenium, and a ductile mixed layer of ruthenium-niobium. With either process, the joining of rhenium to a niobium is accomplished in a manner that minimizes the formation of brittle intermetallics.

Various modifications and alterations to the above described preferred embodiment will be apparent to those skilled in the art. Accordingly, this description of the invention should be considered exemplary and not as limiting the scope and spirit of the invention as set forth in the following claims. 

What is claimed is:
 1. An article comprising:a first layer of rhenium; a second layer from the group consisting of niobium and niobium-based alloys; a bond layer disposed between said first and second layers for bonding said layers together, said bond layer comprising a first portion of mixed ruthenium-rhenium adjacent to said first layer, a second portion adjacent to said first portion comprised of a gold-nickel combination, a gold-palladium-nickel combination, BAu-5, or a nickel-palladium combination, and a third portion of mixed ruthenium-niobium adjacent to said second layer. 